Photosensitive elements for image-reproduction are well-known in the graphics arts industry. Elements are exposed to actinic radiation through an image-bearing transparency, such as a color separation transparency, to produce an image that is either a positive or negative of the transparency. After imagewise exposure, the photosensitive elements are developed by washing out the soluble image areas, toning with a colorant, peeling apart, or combinations of these techniques.
Peel-apart photosensitive elements, which do not require a solvent for their development, are disclosed in Cohen, U.S. Pat. Nos. 4,174,216 and 4,247,619. These elements comprise, in order a strippable coversheet, a photoadherent photohardenable layer, a tonable contiguous layer, and a support. The element is imagewise exposed through the coversheet, and the coversheet is subsequently peeled off. The exposed areas of the photosensitive layer adhere to the coversheet and are removed, revealing the tonable contiguous layer. The unexposed areas of the photosensitive layer which remain on the contiguous layer do not accept toner. The revealed contiguous layer is toned in a succeeding step to produce a negative image of the transparency used for exposure. The toned image may be used as a single-color surprint proof.
The process described above can be used to produce a multicolored image. First, the support is removed from a photosensitive element thereby revealing the contiguous layer. The contiguous layer of the resulting element is adhered to the exposed and peeled-apart photohardenable layer of a previously formed colored image. The resulting element is then imagewise exposed through the coversheet, peeled-apart, and toned with a different colored toner to produce a two-color image. This sequence of steps may be repeated as many times as desired to produce a multicolored surprint proof.
The elements disclosed by Cohen have numerous advantages for the production of colored images. However, imperfections known as "pickoff defects" can occur during image formation. In addition, these elements may have a higher dot gain than the press print. A high dot gain is unsuitable in the printing industry.
Pickoff defects occur when portions of the contiguous layer adhere to the exposed photohardenable layer and are removed with the photohardenable layer when the exposed element is peeled apart. Regions which should accept toner are not able to do so because the tonable contiguous layer has been removed. Also, in the production of multicolored images, the contiguous layer from an earlier formed colored image may be revealed and accept toner during the toning process. Consequently, pickoff defects have a deleterious effect on the image and may make it unsuitable for use as a prepress proof.
The production and mounting of printing plates is expensive and time consuming. Thus, it is essential that a proof accurately predict the appearance and quality of the image obtained by printing. The dots on a halftone press print are larger than the dots on the corresponding halftone separation transparency. This phenomenon is known as dot gain. A discussion of dot gain and its causes can be found in Principles of Color Proofing, by Michael H. Bruno, GAMA Communications, Salem, NH, 1986, pp 84-87. For a prepress proof to be an acceptable representation of the press print, it must accurately duplicate the dot gain of the press print. The elements described by Cohen generally produce too much dot gain and thus, the resulting proof does not completely simulate the press print.
Precolored peel apart photosensitive elements, in which the photohardenable layer comprises a colorant, are disclosed in Taylor, U.S. Pat. No. 4,489,154, and in Choi, U.S. Pat. No. 5,001,036. In certain instances, these elements have an advantage over tonable elements because it is unnecessary to apply a colorant to the element after it has been exposed and peeled apart. Although these elements have been used to produce overlay proofs, it has not been possible to use them to produce multicolor surprint proofs due to pickoff defects.
Accordingly, a need exists for a photosensitive element which does not have pickoff defects and which more accurately duplicates the dot gain of the printing press.